1. Field of the Invention
The invention relates to a pixel circuit, and more particularly, to a pixel circuit capable of compensating for transistor threshold voltage drift.
2. Description of the Related Art
Due to the use of Thin Film Transistor-Active Matrix Organic Light Emitting Diodes (TFT-AMOLEDs), the current display device has a low manufacturing cost, high response speed (more than a hundred times that of traditional LCD displays), low power consumption, a huge operating temperature range, as well as a light weight, etc., and therefore, use of TFT-AMOLEDs has become mainstream.
There are two ways of manufacturing the TFT-AMOLED, one is by using the Low Temperature Poly-silicon (LTPS) TFT technology and another one is by using the Amorphous Silicon (a-Si) TFT technology. When driving the TFT, the LTPS technology usually adopts P type transistors as the driving TFT, and the a-Si usually adopts N type transistors as the driving TFT. The a-Si technology results in a comparably better thin-film transistor uniformity, as well as lower production costs. However, the disadvantage of using the N type driving TFT is that the threshold voltage of transistors may drift after being used for a period of time. Therefore, even after applying the same driving voltage, after being used for a period of time, the driving TFT is unable to output the same driving current as initially, causing some lines to undesirably become darker or brighter than it should be. This is called the MURA effect. Another disadvantage, is that the N type driving TFT is used with an inverted OLDE, and the fabrication of the inverted OLDE is more complex as compared to a normal OLED.
Therefore, a novel pixel circuit using the N type driving TFT with the normal OLED, capable of compensating for the threshold voltage drift, is highly required.